Olfactory sensation profoundly influences a person's quality of life. In addition to the aesthetic aspects of life associated with olfactory sensation, approximately 2 million Americans experience some type of olfactory dysfunction. Studies show that olfactory dysfunction affects at least 1% of the population under the age of 65 years, and well over 50% of the population older than 65 years. The sense of smell determines the flavor of foods and beverages and serves as an early warning system for the detection of environmental hazards, such as spoiled food, leaking natural gas, smoke, or airborne pollutants. The losses or distortions of smell sensation can adversely influence food preference, food intake and appetite.
Olfactory disorders are classified as follows: 1) anosmia: inability to detect qualitative olfactory sensations (e.g., absence of smell function), 2) partial anosmia: ability to perceive some, but not all, odorants, 3) hyposmia or microsmia: decreased sensitivity to odorants, 4) hyperosmia: abnormally acute smell function, 5) dysosmia (cacosmia or parosmia): distorted or perverted smell perception or odorant stimulation, 6) phantosmia: dysosmic sensation perceived in the absence of an odor stimulus (a.k.a. olfactory hallucination), and 7) olfactory agnosia: inability to recognize an odor sensation.
Olfactory dysfunction is further classified as 1) conductive or transport impairments from obstruction of nasal passages (e.g., chronic nasal inflammation, polyposis, etc.), 2) sensorineural impairments from damage to neuroepithelium (e.g., viral infection, airborne toxins, etc.), 3) central olfactory neural impairment from central nervous system damage (e.g., tumors, masses impacting on olfactory tract, neurodegenerative disorders, etc.). These categories are not mutually exclusive. For example, viruses can cause damage to the olfactory neuroepithelium and they may also be transported into the central nervous system via the olfactory nerve causing damage to the central elements of the olfactory system.
Smelling abilities are initially determined by neurons in the olfactory epithelium, the olfactory sensory neurons (hereinafter “olfactory neurons). In olfactory neurons, odorant receptor (hereinafter “OR”) proteins, members of the G-protein coupled receptor (hereinafter “GPCR”) superfamily, are synthesized in the endoplasmic reticulum, transported, and eventually concentrated at the cell surface membrane of the cilia at the tip of the dendrite. Considering that ORs have roles in target recognition of developing olfactory axons, OR proteins are also present at axon terminals (see, e.g., Mombaerts, P., (1996) Cell 87, 675-686; Wang, F., et al. (1998) Cell 93, 47-60; each herein incorporated by reference in their entireties). In rodents, odorants are transduced by as many as 1000 different ORs encoded by a multigene family (see, e.g., Axel, R. (1995) Sci Am 1273, 154-159; Buck, L., and Axel, R. (1991) Cell 65, 175-187; Firestein, S. (2001) Nature 413, 211-218; Mombaerts, P. (1999) Annu Rev Neurosci 22, 487-509; Young, J. M., et al., (2002) Hum Mol Genet. 11, 535-546; Zhang, X., and Firestein, S. (2002) Nat Neurosci 5, 124-133; each herein incorporated by reference in their entirety). Each olfactory neuron expresses only one type of the OR, forming the cellular basis of odorant discrimination by olfactory neurons (see, e.g., Lewcock, J. W., and Reed, R. R. (2004) Proc Natl Acad Sci USA; Malnic, B., et al., (1999) Cell 96, 713-723; Serizawa, S., et al., (2003) Science 302, 2088-2094; each herein incorporated by reference in their entirety).
What is needed is a better understanding of olfactory sensation. What is further needed is a better understanding of odorant receptor function.